The invention relates to a an ultrasonic method and an ultrasonic system for determining local propagation velocity of transient shear waves in a tissue, for displaying a sequence of velocity images of the transient shear waves and for determining tissue elasticity information.
The invention finds its application in using this information as a tool to diagnose abnormalities, such as tumors or edemas, in a patient tissue. These abnormalities are known to show changes of their mechanical properties with respect to sound background tissue. Shear wave propagation information permits of localizing said abnormalities.
A method for determining tissue elasticity in various parts of a body is already known from Sarvazyan, U.S. Pat. No. 5,606,971. According to this known method, ultrasonic waves are focussed at different location of a tissue, using a focused ultrasonic source that transmits said ultrasonic waves to its focal region. The focused ultrasonic source is preferably an ultrasound transducer of the phased array kind. Said focussed ultrasonic waves are amplitude modulated for generating shear waves at said different locations of the tissue. Said shear waves are further detected by measuring their amplitude and phase on the surface of the tissue. At least one propagation parameter of the shear waves in the tissue is determined from the phase and amplitude measures such as shear wave velocity, attenuation coefficient, amplitude and velocity of shear displacement of tissue particles in the propagating shear wave. A calculation, based on these measures, is performed and at least one mechanical parameter of tissue is determined such as the shear elasticity modules, Young modulus, dynamic shear viscosity, using known relations. The steps of the method are repeated for all amplitude modulated focused ultrasound waves, which are focused at said various locations. The calculated values of dynamic shear viscosity and elasticity modulus are displayed in function of the coordinates of said locations.
This known method of producing shear waves necessitates the use of focussed ultrasonic sources such as phased array transducers, for transmitting a great amount of ultrasonic energy to the locations where shear waves are produced in the tissue. Focussed ultrasonic sources may have destructive effects on the patient tissue due to the necessary amount of ultrasonic energy that is locally applied.
The present invention has for an object to provide a method, and a system to carry out the method, which do not use a focused ultrasonic source, in order to avoid possible secondary effects in the patient tissue. According to the invention, shear waves are generated in a tissue using an external mechanical vibration source. The shear waves produce displacements of tissue particles. According to the invention, a standard ultrasonic diagnostic system measures the velocity of the tissue particles and the velocity of the front of the shear waves. This ultrasonic diagnostic system also has means to display a sequence of images of the shear wave front propagation.
A problem is that the shear waves propagate at about 1 m. per s. over several centimeters in the tissue, for instance 4 cm. In this case, the propagation time is about 40 ms (milliseconds). This velocity is much too high and the propagation time delay much too small to permit of visualizing the effect of shear waves on the tissue using a standard sequence of ultrasonic images produced by a standard ultrasonic diagnostic imaging system. The image frame rate of such a system is not adapted to visualizing the shear wave propagation because it is of the order of 15 images per second whereas an image frame rate of about 1 image per ms (1000 images per second) is needed for shear wave visualization.
The present invention provides, as claimed in claim 1, an ultrasonic diagnostic imaging method for determining propagation parameters of transient shear wave front, comprising steps of forming transient shear waves in a tissue, acquiring ultrasonic image data (S,S*) of the tissue, along image lines, during a time delay for a transient shear wave front to propagate over a depth (z) in said tissue, estimating the tissue velocity (V) for each line, constructing a tissue velocity image sequence [I(V)] from the ultrasonic data (S,S*) and the tissue velocities (V) on the lines, and deriving the velocities (CSW) of the shear wave front at instants of the sequence. The invention also provides an ultrasonic diagnostic imaging method, as claimed in claim 7, for determining tissue local mechanical parameters of a tissue from the transient shear wave front velocity (CSW).
The present invention further provides a system, as claimed in claim 8 for carrying out said methods.
The invention allows the visualization of sequences of tissue moving under the influence of shear wave, using a standard ultrasound diagnostic imaging system with a standard transducer emitting and receiving standard ultrasound and echo signals respectively. The invention further allows the localization of tissue regions having contrasting mechanical properties with respect to a background, and the determination of the mechanical parameters of said tissue regions.